Police said the shooter appeared to have been the victim of bullying and had been receiving psychological treatment

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Policemen patrol the Stachus square following shootings at a shopping mall on July 22, 2016 in Munich.

The U.S. psychologist who wrote a book about school shootings that investigators found in the Munich gunman's room says researching other mass killers can be a warning sign since young gunmen often are looking for role models — but it's just one of many possible indicators.

Peter Langman, author of two books about school shootings, told The Associated Press by telephone from Allentown, Pennsylvania on Saturday, that juvenile killers "more frequently study other shooters."

Police in Munich said the 18-year-old German-Iranian gunman had a German translation of Langman's 2010 book "Why Kids Kill: Inside the Minds of School Shooters," along with materials relating to a 2009 school shooting in Germany and the bomb-and-gun attacks in Norway by Anders Behring Breivik, who killed 77 people five years ago Friday.

"Younger shooters in particular, meaning adolescents into their 20s, often research other shooters and find a role model. That is not something you see with the older shooters," Langman said.

The shooter was found dead of an apparent self-inflicted gunshot wound several hours after the Friday evening attack that left nine others dead, most of them teens. Police are investigating whether the killer was behind a hacked Facebook account that sought to lure other youths with free giveaways to a McDonald's that was targeted. They also said the shooter appeared to have been the victim of bullying and had been receiving psychological treatment.

Langman said there are often a lot of warning signs before a shooting attack, and that "it is never one thing; it is always a combination of multiple factors."

"He may have been bullied. Whether or not that was a factor, we don't know," he said. "Bullying is much less significant than people tend to think it is. Not that it is never a factor, but in my research it is not a prominent a factor that it is widely believed to be. "

In some cases "people simply announce what they are going to do," he said.

"A direct example would be someone announcing to an intended victim or to a friend, 'I am going to bring a gun to school and kill people.' Sometimes it is that direct, but people don't take it seriously," he said.

Langman said less direct hints at troubled minds would be "someone commenting on a school shooting and saying, 'That is pretty cool, someone should do that here.'"